Alberto Curcella2 Romain Bernard2 Yves Borensztein2 Michele Lazzeri1 Silvia Pandolfi2 Geoffroy Prevot2

2, Sorbonne Université-INSP, Paris, , France
1, Sorbonne Université-IMPMC, Paris, , France

Using real-time in-situ scanning tunneling microscopy, Auger electron spectroscopy and density functional theory simulations, we have studied the growth of Si films on Ag(111) beyond the silicene monolayer, evidencing the existence of metastable phases and an original growth mechanism. Above monolayer Si coverage, a first (√3×√3)R30° structure forms, which we identify as a Ag-free Si bilayer with additional Si adatoms. With further deposition, this structure is replaced by a distinct bilayer structure covered by Si trimers and Ag atoms. The formation of these bilayers follows a counterintuitive dynamics: they are partially inserted within the Ag substrate and form by expelling, from the underlying substrate, atoms that reinsert below the adjacent silicene layer. The growth is therefore characterized by an unexpected "surfactant competition" between Ag and silicene: while silicene is a metastable surfactant for the Ag(111) surface, Ag plays the role of a surfactant for thicker diamond-like Si islands. This mechanism also reproduces the evolution of the Si and Ag Auger intensities during growth. Our conclusion is that, due to the high kinetic-barrier for the growth of thicker layers, the silicene monolayer is a remarkably stable structure, in spite of being thermodynamically unfavoured.